1. Technical Field
The present invention relates to a pixel circuit, an electro-optical device of a digital driving type (for example, a liquid crystal display device employing a subfield driving type), and an electronic apparatus.
2. Related Art
For example, as a driving method for a liquid crystal display device, there are an analog driving type (a method in which an analog voltage corresponding to a display gray scale is applied to a data line) and a digital driving type (a driving method in which there are only two values including an ON level and an OFF level as voltage levels needed for driving a pixel). In the analog driving type, non-uniformity of display can be easily generated due to the non-uniformity of characteristics of elements (a D/A converter, an operational amplifier, a wiring, and the like) configuring the circuit. On the other hand, in the digital driving type, used driving voltages (writing voltages for a pixel) are binary, and accordingly, it is easy to implement image display having higher definition.
As the digital driving type, a subfield driving type in which one field is divided into a plurality of subfields on a time axis and an ON voltage or an OFF voltage is applied to each subfield in accordance with the gray scale of each pixel has been proposed (for example, see JP-A-2003-114661). In this subfield driving type, a voltage (effective voltage) supplied to a liquid crystal is changed by controlling not a voltage level but an application time interval of a voltage pulse, and thereby the transmissivity of the liquid crystal is controlled.
As a pixel circuit that can be used in the liquid crystal display device of the digital driving type, for example, there are a DRAM-type pixel circuit (a general pixel circuit having a pixel transistor and a holding capacitor) and a pixel circuit (a RAM-type pixel circuit) that has a RAM (a memory circuit that maintains “1” or “0” by using positive feedback) instead of the holding capacitor.
The DRAM-type pixel circuit has an advantage that the circuit configuration thereof is simple. However, in the DRAM-type pixel circuit, a change (for example, see JP-A-5-224235) in the maintained voltage caused by feed-through or a change in the maintained voltage caused by a leak current can easily occur. Accordingly, for performing high-definition display, there is limit in the DRAM-type pixel circuit.
In addition, as the RAM-type pixel circuit, for example, a pixel circuit that uses an SRAM has been proposed (see JP-A-2005-258007). When the pixel circuit disclosed in JP-A-2005-258007 is used, high-definition image display can be performed. However, in such a case, the circuit configuration is complicated. Accordingly, an increase in the size of the pixel circuit and an increase in the power consumption of the pixel circuit are inevitable.
In the liquid crystal display device of the digital driving type, when a RAM-type pixel circuit is used, the circuit configuration is complicated, and thereby an increase in the size of the pixel circuit and an increase in the power consumption of the pixel circuit occur. Accordingly, it is difficult to implement high-definition image display while miniaturization and low power consumption of the liquid crystal display device are achieved.